Field of the Invention
The invention relates to a switched capacitor network having a switch device which charges two first capacitors of the same capacitance alternatingly from a signal voltage source with polarities that are opposite relative to a ground potential and then discharges them each through a respective input circuit of a differential amplifier, and in synchronism therewith it alternatingly charges two second capacitors of the same capacitance and in phase opposition with one another from a reference voltage source with the same polarities relative to the ground potential and then discharges them each through a respective input circuit of the differential amplifier.
Active networks, to be realized, typically require an operational amplifier as an active component and capacitors and resistors as passive elements. The frequency dependency of the network is determined by the capacitors and resistors being used. Conversely, in switched capacitor networks, resistors are simulated by switched capacitors, and there is a linear relationship between the switching frequency and the equivalent electric conductance or guide value, corresponding to an ohmic resistor. The frequency dependency of the network can thus be varied in a simple way by varying the switch frequency.
Typically, the capacitors of a switched capacitor network are switched in such a way that they are each charged in a switching phase and discharged again in an ensuing switching phase. Instead of the discharge, a charge reversal can also ensue, which is equivalent to discharging with subsequent charging having opposite polarity. The onset of the charging or charge reversal process represents a major load for the charging voltage source, since the capacitor briefly forms a short circuit, and the current is primarily limited only by the internal resistance of the voltage source and the contact resistance of the switch device. The consequence is initially a voltage dip at the voltage source and subsequently a transient effect, which is determined essentially by the capacitance of the capacitor, the internal resistance of the voltage source, and the contact resistance of the switch device. That can cause inadequate charging of the respective capacitor in the corresponding switching phase, and resultant disruptions in the entire network.
If a switched capacitor network has an (additional) signal path with a further capacitor, which is coupled to the already existing path and carried to a (differential) amplifier, then the charging of the first capacitor can also be dependent on the signal in the signal path. Such a network is defined, for instance, by a sigma-delta modulator described in Published European Application No. 0 396 786 A1, corresponding to U.S. Pat. No. 4,999,634.
In it, two identical capacitors in the signal path are alternatingly charged from a signal voltage source that is symmetrical relative to a ground potential and then are discharged through corresponding differential inputs of a differential amplifier. A feedback path is coupled to the signal path and essentially includes a digital/analog converter triggered by the output signal of the sigma-delta modulator. The digital/analog converter is likewise differentially constructed and therefore also requires differential reference voltages. However, instead of a bipolar voltage source only a unipolar voltage source is used, and the two capacitors of the digital/analog converter are therefore charged and discharged in phase opposition with one another in the feedback path.
However, it is problematical in that case that the reference voltage source used to charge the capacitors in the feedback path typically includes a closed-loop control circuit, such as a suitably wired operational amplifier. In a first switching phase, only the signal is present at the coupling point of the signal path and the feedback path, and the operational amplifier of a following integrator is decoupled. In that phase, one of the two capacitors in the feedback path is charged, but without major effects on the associated coupling point. In the ensuing second switching phase, the other capacitor in the feedback path is charged, specifically through the input circuit of the following operational amplifier, which is wired as an integrator, to which the capacitors in the signal path are also connected. Consequently, the operational amplifier stabilizes the nodes relative to the ground potential, as a function of the signal. That process is copied to the reference voltage source, through the connector capacitor in the feedback path. As a rule, however, the operational amplifier of the reference voltage path has a substantially narrower bandwidth than the operational amplifier of the integrator, so that the disturbances in the reference voltage source caused by the signal cannot be stabilized.